Rotating optical joint

ABSTRACT

A rotating optical joint has two organs which are able to rotate independently of one other on a common axis ( 14 ). Collimators ( 18, 20 ) are mounted directly opposite one another on the organs, in an arrangement such that they permanently provide a variation of the power of the transmitted signal of less than 25%, during relative rotation of the said organs. The total number of collimators ( 18, 20 ) is preferably less than or equal to eight. Advantageously, three collimators ( 20 ) are mounted on one of the organs and four collimators ( 18 ) are mounted on the other organ.

TECHNICAL FIELD

The invention concerns a rotating optical joint intended to transmitdata in an optical form between two organs able to rotate independentlyof one another on a common axis.

More precisely, the invention concerns a rotating optical joint designedto provide transmission of data in a manner which is offset from thecommon axis of the said organs.

Such joints can be used in many different technical fields, such asradar or tank turrets, helicopter rotors, etc.

CURRENT STATE OF TECHNOLOGY

To transmit data between two organs which must rotate independently ofone another, one traditional solution consists in using rotatingelectrical joints.

In joints of this type, electrical signals are transmitted by mechanicalparts which rub against one another.

Rotating electrical joints do however have major disadvantages. Amongthese disadvantages one must mention notably the wear and tear of theparts due to the mechanical contact, which requires regular servicing,the appearance of interference caused by the rubbing of the parts incontact, and the limited nature of the bandwidth of such a joint (of theorder of some ten to some hundred megahertz).

Rotating optical joints enable these various problems to be resolved.They transmit the optical signals without contact, they are highlyinsensitive to interference and they allow a much higher transmissionrate than rotating electrical joints.

Most rotating optical joints transmit signals in the axis common to thetwo organs which are to rotate independently of one another. They arethen called “on-line” joints. In this case, two optical fibres or otherlight conductors are positioned facing one another on the commonrotational axis.

The main disadvantage of these rotating optical joints is that theygenerally provide only a single transmission channel or a single opticalchannel.

In addition, they require that the data is transmitted in the commonrotational axis, which is not satisfactory in certain specialapplications.

In the article by N. Lewis et al. “A four channel bidirectional datalink using wavelength division multiplexing”, in the periodical“Proceeding of the SPIE”, published by “The International Society forOptical Engineering”, vol. 574, 1985, pages 47 to 54, it was proposed toovercome the first disadvantage by using multiplexing systems. In thiscase, several transmission channels or optical channels transmitsimultaneously in the axis of the joint.

The major disadvantage of this solution is that multiplexing requireselectronics and thus an energetic contribution. In addition, data isalways transmitted in the rotational axis.

Over recent years, various solutions have been proposed to transmit datain an manner offset in relation to the axis of a rotating optical joint.Joints of this type, frequently called “off-axis” joints, enable thesystem's axis to be left free, whilst transmitting the data in the crownof the joint. In addition, these joints are by their naturemulti-channel. They thus allow several transmission channels, when thisis required.

A first type of “off-axis” joint is described in the documents U.S. Pat.No. 4,525,025 and U.S. Pat. No. 5,991,478. A rotor is positionedcoaxially inside a stator. One or more prisms or optical fibres mountedin the stator emit light approximately tangentially in the direction ofthe stator's reflecting cylindrical inner wall. The light reflected onthis surface is captured by one or more receptor prisms or fibresmounted in the rotor. Use of prisms or fibres oriented in reversedirections makes the system bidirectional.

This type of joint has various disadvantages. If optical fibres are usedthe reception system is practically in contact with the reflecting innerwall. The latter may thus be damaged in the event of an impact. Inaddition, the transmission rate is limited to 50 Mbit/s for joints of adiameter of between approximately 25 and 30 cm.

Optical joints in which light signals are transmitted using prisms, inaccordance with document U.S. Pat. No. 5,991,478, also have thedisadvantage of being costly, due to the number of components which theycontain. Moreover, the output signal is not an optical signal, but anelectrical signal. Finally, the receptor requires an energeticcontribution to be polarised.

Document U.S. Pat. No. 4,753,506 proposes a rotating optical joint ofthe “off-axis” type in which two cylindrical parts rotate in relation toone another in a common axis.

In a first configuration, one of the parts carries an emitter andseveral prisms distributed circumferentially so as to receive in turnthe light beam from the emitter. The other part carries a receptorfacing the prisms. Each of the prisms directs a conical light beam withan ellipsoidal base to the other part, so as to provide total coverageof the zone through which the receptor moves.

In another configuration, one of the parts carries a single emitter andthe other part carries several receptors. In this case, the emitterdirects a conical light beam with an ellipsoidal base directly to thereceptors.

This known arrangement has the disadvantages that the output signal iselectrical and the power must be high.

Finally, we are also familiar with the article “Proof of concept modelsof a multichannel off-axis passive bi-directional fiber optic rotaryjoint”, by Koch W W et al., published in the periodical “Proceedings ofthe SPIE”, “The International Society for Optical Engineering”, volume931, 1988, pages 94 to 97, a rotating optical joint of the “off-axis”type. In this joint, an intermediate optical element incorporating abeam of optical fibres, is interposed axially between a rotor and astator. The latter are both fitted with optical fibres associated withcollimation lenses directed at this intermediate element and regularlydistributed around the axis common to the rotor and the stator. Amechanism makes the intermediate element rotate in the same direction asthe rotor, at an angular speed equal to half the speed of the rotor.

Documents U.S. Pat. No. 5,157,745 and U.S. Pat. No. 5,371,814 proposecomparable arrangements, in which the beam of fibres of the intermediateelement is replaced by a Dove prism.

This type of rotating optical joint with an intermediate element has themajor disadvantage that it requires very substantial adjustments,implementation of which is particularly delicate and difficult.

ACCOUNT OF THE INVENTION

The purpose of the invention is precisely a rotary optical joint of the“off-axis” type, preferably bidirectional, which does not have thedisadvantages of joints of this type in the prior art, and which notablyallows transmission of optical signals with high transmission rates (2.5Gbit/s), in a relatively simple and inexpensive manner, and leavingfree, if necessary, the region where the joint's axis is located.

In accordance with the invention, this goal is attained by means of arotating optical joint, comprising a first organ and a second organ ableto rotate independently of one another on a common axis, and at leastone transmission channel of an optical signal between the said organs,with each transmission channel comprising first collimators of the lightemitted from a first optical fibre, mounted on the first organ andproducing ray beams approximately parallel to the common axis and offsetin relation to this axis, and second collimators able to focus in asecond optical fibre the collimated light from the first organ andmounted on the second organ, in which the first and second collimatorsare positioned approximately at an identical distance from the saidcommon axis, characterised in that the first and second collimators arelocated directly opposite one another, and in such numbers and arrangedsuch that they provide a permanent variation of the power of thetransmitted optical signal of less than 25%, during relative rotation ofthe said organs.

The particularly simple arrangement defined above allows bidirectionaltransmission of optical signals, whilst leaving the central part of thejoint totally free, when necessary. Thus, the first and second organscan easily be hollow along the said common axis, when required by theapplication.

In addition, it is possible to transmit optical signals with a hightransmission-rate, of the order of 2.5 Gbit/s, which enables this typeof joint to be used in the most demanding applications.

Each of the two organs of a rotating optical joint comprises a number ofcollimators and a number of optical fibre couplers. The function of thecollimators is to transform the divergent beam into a collimated beam,in the beam's propagation direction. The optical fibre couplers, fortheir part, are used for dividing and subsequently for regrouping thebeams.

In the preferred embodiments of the invention, the total number of thefirst and second collimators is less than or equal to eight. This smallnumber enables the number of fibre optical couplers used in each of thetwo organs to divide or regroup the beams from the various collimatorsof a given organ, which is the cause of most of the intrinsic losses inthe system, to be kept small.

In another preferred embodiment of the invention, the number ofcollimators on each organ is even, and this number is respectively equalto four for the first collimators and to two for the second collimators.This arrangement is particularly advantageous since it allows onlyexisting couplers to be used at one input and two outputs, and viceversa.

In this case, the first four collimators are preferably regularly spacedaround the said common axis and the two collimators are arranged suchthat when one of them is aligned with one of the first collimators, theother is located approximately tangentially between two other firstcollimators.

In another embodiment of the invention, the number of the first andsecond collimators is respectively equal to four and to three. Comparedto the previous embodiment, this arrangement is more favourable since itenables the phenomenon of power variation to be reduced during therotation of an organ (this is then called the transmitted signalmodulation phenomenon). However, it uses more complex optical fibrecoupler technologies.

Advantageously, each of the first and second collimators hasapproximately the same section.

In another aspect of the invention, the transmission channel comprises,inside each of the first and second organs, emitting and receivingoptical fibres linked to each of the first and second collimators byoptical couplers enabling beams to be separated/recombined.

In the preferred embodiments of the invention, the optical fibres aremultimode fibres.

BRIEF DESCRIPTION OF ILLUSTRATIONS

We shall now describe a preferred embodiment of the invention, referringto the annexed illustrations, in which:

FIG. 1 is a side view, as a section, which represents a rotating opticaljoint constructed in accordance with the invention;

FIG. 2 is a section which illustrates diagrammatically the positioningof the collimators in the optical joint in FIG. 1;

FIG. 3 is an outline diagram of the optical joint in FIGS. 1 and 2,which shows notably the optical fibre couplings and the path of thelight signals; and

FIG. 4 represents the change in modulation (as a percentage) of thelight signals inside the optical joint in the invention, as a functionof the relative angular rotation (in degrees) of the two organsconstituting it, in the case of a joint with two and four collimators.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

In FIG. 1 we have represented the main part of a rotating optical jointof the “off-axis” type in the invention. As this diagram illustrates,the joint comprises a first organ 10 and a second organ 12, with acommon axis 14. Organs 10 and 12 are able to rotate independently of oneanother on this common axis 14. To this end, bearings 16 can notably beinterposed between these organs 10 and 12.

As is illustrated equally by FIG. 1, the first organ 10 comprises an endface 10 a located opposite an end face 12 a of the second organ 12.These end faces 10 a and 12 a are perpendicular to axis 14 which iscommon to both organs.

In the preferred embodiment of the invention illustrated as an examplein FIGS. 1 and 2, the first organ 10 supports four first collimators 18,and the second organ 12 supports two second collimators 20. Morespecifically, the first collimators 18 extend outwards from the end face10 a of the first organ 10 and the second collimators 20 extend outwardsfrom the end face 12 a of the second organ 12. Collimators 18 and 20 arethus located directly opposite one other during relative rotation oforgans 10 and 12.

Each of collimators 18 and 20 have an optical axis oriented in parallelto the common axis 14 and positioned at an identical, non-zero distancefrom this common axis. In other words, the optical axes of collimators18 and 20 are all offset by an identical distance compared to axis 14which is common to organs 10 and 12.

In addition, the optical axes of the first collimators 18 are regularlyspaced in relation to one another, which means that each of theseoptical axes is offset by an angle of 90° to the optical axes of thefirst adjacent collimators 18.

In addition, as FIG. 2 shows more specifically, the optical axes of thesecond collimators 20 are offset angularly such that, when one of theseaxes is aligned with the optical axis of one of the first collimators18, the optical axis of the other second collimator 20 is positionedangularly at an equal distance between two others of the firstcollimators 18. More precisely, distance d between the common axis 14and each of the optical axes of the collimators 18 and 20, together withsection s of these various collimators, are determined such that theother second collimator 20 is then approximately tangential to the twoother first collimators.

As an example in no way limiting of the scope of the invention, thisarrangement can notably be obtained by giving distance d a value equalto 21 mm and using collimators with a useful diameter of 16 mm.

Comparable properties could, of course, be obtained by increasing orreducing these two magnitudes simultaneously, for example in order toobtain a central passage 22 of greater or lesser size, respectively.

It should be noted that for a given value of distance d, the geometricalarrangement defined above constitutes a lower limit for section s. Inother words, the section of the collimators may be such that, when oneof the second collimators 20 is aligned with one of the firstcollimators 18, the other second collimator 20 is positioned between twoother collimators 18 and partially straddles the latter when the jointis seen at the end.

Collimators 18 and 20 are constituted by all optical systems able to bepositioned at the end of optical fibres to emit or receive a parallellight beam. These optical systems, which are well known to those skilledin the art, habitually comprise a lens or sets of lenses allowing adivergent beam to be transformed into a collimated beam. We can mentionas non-limiting examples lenses with index gradients of the GRIN typeand aspherical lenses.

As FIG. 3 illustrates more specifically, the rotating optical joint inthe invention also comprises a number of optical fibres and opticalfibre couplers mounted on each of organs 10 and 12.

Thus, in the embodiment represented which concerns an optical joint ableto operate in a bi-directional manner, the first organ 10 comprisesoptical fibres and optical fibre couplers enabling each of the firstcollimators 18 to be linked firstly to a source 28 able to emit lightsignals, and also to a receiver 30 of light signals. In comparablefashion, the second organ 12 comprises optical fibres and optical fibrecouplers enabling each of the second collimators 20 to be linked firstlyto a source 24 able to emit light signals, and also to a receiver 26 oflight signals.

Sources 24 and 28 are, for example, laser diodes emitting around 1325mm. The receivers depend on the envisaged application.

More precisely, a first optical fibre 32 links source 24 to a firstoptical coupler in X 34 inside the second organ 12. In addition, twooptical fibres 36 link the first coupler in X 34 to each of the secondcollimators 20. Finally, another optical fibre 38 links the firstcoupler in X 34 to the receiver 26.

In comparable fashion, in the first organ 10 an optical fibre 40 linksthe source 28 to a second optical coupler in X 42. Two optical fibres 44also link the second optical coupler in X 42 respectively to a thirdoptical coupler in Y 46 and to a fourth optical coupler in Y 48. Twoother optical fibres 50 link the third optical coupler in Y 46 to two ofthe first collimators 18, and in addition two other optical fibres 52link the fourth optical coupler in Y 48 to the two other firstcollimators 18. Finally, a final optical fibre 54 links the secondoptical coupler in X 42 to the receiver 30 of the first organ 10.

All the couplers in the optical joint are globally balanced, such thatthe power of a signal is globally divided or multiplied by two whenpassing through each of them, give or take intrinsic losses.

The arrangement described above enables the coupler to operate in abi-directional manner.

Thus, optical signals emitted by source 24 of the second organ 12 areconveyed by fibre 32 to the coupler 34, which divide the signals intotwo parts conveyed respectively by fibres 36 to each of the secondcollimators 20. The optical signals emitted from the latter arerecovered by the first collimators 18, with a modulation which shall bedescribed in detail subsequently. The said signals then pass to thecouplers 46 and 48 through optical fibres 50 and 52. On output fromcouplers 50 and 52, they are conveyed to coupler 42 by optical fibres44, before being transmitted to receiver 30 by optical fibre 54.

In comparable fashion, optical signals emitted by source 28 associatedwith the first organ 10 are transmitted to receiver 26 of the secondorgan 12 by following a reverse path.

In the rotating optical joint in the invention, the various opticalfibres are advantageously multimode fibres, with a digital openingchosen such that it enables the signal modulation phenomenon to belimited whilst one organ is rotating relative to the other.

As shown by the graph in FIG. 4, the consequence of the arrangement ofthe collimators 18 and 20 in the invention is a cyclic variation of themodulation of the transmitted signal (as a % of half the signal), as afunction of the relative angular rotation between organs 10 and 12 (indegrees).

More precisely, in the case of a configuration in which the light isconveyed by an optical fibre from the two collimators 20 to the fourcollimators 18, and given the usual losses of “X” and “Y” fibrecouplers, the energy recovered in organ 10 is at least equal to 34% ofthe energy injected in organ 12 and its variation during relativerotation of organs 10 and 12 is less than 25% of the amplitude of therecovered signal.

In the case of a configuration in which the light is conveyed by anoptical fibre from the four collimators 18 to the two collimators 20,and given the usual losses of “X” and “Y” fibre couplers, the energyrecovered in organ 12 is at least equal to 17% of the energy injected inorgan 10 and its variation during relative rotation of organs 10 and 12is less than 25% of the amplitude of the recovered signal.

It should be noted that the total losses are the same in bothdirections, as a consequence of the bi-directional nature of the jointand the principle of reverse return of light.

In addition to this loss, due essentially to the geometry of therotating optical joint in the invention, there are optical lossesinherent to the presence of optical elements such as collimators 18 and20 and couplers 34, 42, 46 and 48. In the arrangement described above,the overall optical loss of the joint (in terms of power) is estimatedat approximately −12 to −15 dB.

As FIG. 2 notably illustrates, the rotating optical joint in theinvention enables the entire central part of the joint near common axis14 to be kept free, when this is necessary.

Naturally, the invention is not limited to the preferred embodimentdescribed above. Satisfactory results may also be obtained using thefirst and second collimators arranged opposite one another in anotherarrangement, such that their total number remains less than or equal toeight. An arrangement using three first collimators and four secondcollimators is also satisfactory from this standpoint.

1. A rotating optical joint, comprising a first organ and a secondorgan, able to rotate independently of one another on a common axis andat least one channel for transmission of an optical signal between thesaid organs, where each transmission channel contains first collimatorsof the light emitted from a first optical fiber, mounted on the firstorgan and producing light beams approximately parallel to the commonaxis and offset in relation to this axis, and second collimators able tofocus into a second optical fiber the collimated light originating fromthe first organ and mounted on the second organ, which first and secondcollimators are positioned approximately at an identical distance fromthe said common axis, in which the first and second collimators arelocated directly opposite one another, and where their number and theirarrangement are such that their permanent variation of the power of thetransmitted optical signal is less than 25%, during relative rotation ofthe said organs.
 2. A rotating optical joint in claim 1, in which thefirst and second organs are hollow in the said common axis.
 3. Arotating optical joint in claim 1, in which the total number of thefirst and second collimators is less than or equal to eight.
 4. Arotating optical joint in claim 3, in which the number of the first andsecond collimators is respectively equal to four and to two.
 5. Arotating optical joint in claim 4, in which the four first collimatorsare regularly spaced around the said common axis and the two secondcollimators are arranged such that when one of them is aligned with oneof the first collimators, the other is located approximatelytangentially between two other first collimators.
 6. A rotating opticaljoint in claim 3, in which the number of the first and secondcollimators is respectively equal to four and to three.
 7. A rotatingoptical joint in claim 1, in which each of the first and secondcollimators has approximately the same section.
 8. A rotating opticaljoint in claim 1, in which the transmission channel comprises, insideeach of the first and second organs, emitting and receiving opticalfibers linked to each of the first and second collimators by opticalcouplers enabling beams to be separated/recombined.
 9. A rotatingoptical joint in claim 1, in which the optical fibers are multimodefibers.